In recent years, research and development of optical memory devices have been actively carried out to meet various demands, for example, high-density recording, large memory capacity, and high-speed access. In particular, a magneto-optical magnetic thin film using a magnetic film with an axis of easy magnetization perpendicular to a film surface is utilized for a magneto-optical recording medium for use in the optical memory devices.
The following are conventionally well known magneto-optical magnetic thin films. Polycrystal thin films such as MnBi and MnCuBi. Amorphous thin films such as GdTbFeCo, TbFeCo, TbDyFeCo, DyFeCo, GdTbFe, GdDyFe, GdFeCo, TbFe, DyFe, GdFe and GdCo. Compound monocrystal thin films, for example, GIG.
Among these magneto-optical magnetic thin films, the amorphous thin films are suitable for magneto-optical recording media. The reason for this is that the amorphous thin films are practical because the thin films with a large area are manufactured at temperatures in the vicinity of room temperature and that the amorphous thin films are writable and readable, for example, by a practical semiconductor (GaAlAs) laser.
Table 1, below, shows the relation between the amorphous thin film and the Kerr rotation angle (degree) as an index of performance.
TABLE 1 ______________________________________ Material Kerr Rotation Angle (degree) ______________________________________ TbFeCo 0.38 TbDyFeCo 0.37 DyFeCo 0.35 GdTbFeCo 0.40 GdFeCo 0.43 ______________________________________
As shown in Table 1, TbFeCo and GdFeCo exhibited large Kerr rotation angles. Namely, it is found that these amorphous thin films are suitable for use in magneto-optical recording media.
In order to achieve high-density recording, various amorphous thin films with compositions which decrease the writing magnetic field, i.e., increase the perpendicular magnetic anisotropic energy have been proposed.
There is a thin film of amorphous TbFeCo with a composition which gives the highest C/N (carrier-to-noise) ratio even when the writing bit length (recording bit length) is around 0.76 .mu.m. This is achieved because the perpendicular magnetic anisotropic energy of TbFeCo is large.
Table 2, below, shows the relation between the perpendicular magnetic anisotropic energy (Ku) and the C/N ratio of magneto-optical magnetic thin film.
TABLE 2 ______________________________________ Material Ku (erg/cm.sup.3) C/N Ratio (dB) ______________________________________ TbFeCo 6.5 .times. 10.sup.5 49.2 DyFeCo 4.5 .times. 10.sup.5 46.5 GdTbFeCo 3.2 .times. 10.sup.5 46.0 GdFeCo 1.9 .times. 10.sup.5 45.0 ______________________________________
As shown in FIG. 2, TbFeCo which exhibits a Kerr rotation angle smaller than that of GdFeCo gives a greater perpendicular magnetic anisotropic energy and an improved C/N ratio. Thus, in order to improve the C/N ratio, it is necessary to increase the perpendicular magnetic anisotropic energy rather than the Kerr rotation angle shown in FIG. 1. Namely, the value of the perpendicular magnetic anisotropic energy is predominant over the degree of the Kerr rotation angle for improving the C/N ratio.
However, if TbFeCo is used as a magneto-optical magnetic thin film, the price of the magneto-optical recording medium is increased because Tb (terbium) is most expensive among rare-earth elements. Furthermore, if DyFeCo which is inexpensive and has relatively large perpendicular magnetic anisotropic energy is used as the magneto-optical magnetic thin film, the magneto-optical recording medium suffers from another problem that the C/N ratio is impaired.
Then, in order to maintain the C/N ratio at a practical level and to prevent an increase in the price, for example, Japanese Publication for Unexamined Patent Application No. 3-209647 discloses a magneto-optical recording medium using a magneto-optical magnetic thin film of a quaternary alloy including DyFeCo plus Tb.
However, in this magneto-optical recording medium, the composition of TbDyFeCo was arranged so that the coercive force becomes not larger than 5 kOe so as to increase the sensitivity of the recording magnetic field. Therefore, a multiplicity of error occurred after a durability test in which around 10.sup.6 cycles of recording, reproduction and erasing operations were performed on the magneto-optical recording medium.